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DI-88

C

11/05

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Figure 3. Classification Current (Class 1:

R34 = 133

Ω).

Figure 2. Detection Impedance V-I Curve.

VR31 inhibits the classification circuit at input voltages below

11V.ComponentsQ32,Q31andR32forma350µAbiascurrent

source programmed by resistor R33 working in conjunction

with the base-emitter voltage of Q31. Transistor Q33 forms

the classification current source programmed by resistor R34

working in conjunction with the 1.24 V voltage reference U31.

Transistor Q34 disables the classification current source when

Zener diode VR32 conducts (when the input voltage exceeds

approximately 28 V).

Key Design Points

• For Class 0, remove components VR31, R32, R33, R34,

R35, Q31, Q32, Q33, Q34 and U31.

• R34 values: Class 1, R34 = 133 Ω; Class 2, R34 = 68.9 Ω;

Class 3, R34 = 45.3 Ω.

• It is possible to use either bipolar transistor or MOSFET

pass-switches (Q35). A bipolar transistor is less expensive,

but a MOSFET gives higher pass-switch efficiency. See

design idea (DI-70) for details.

• Thebiascurrentsource(Q31)isusedtoallowtheclassification

current source to be turned-off for minimal power loss once

input voltage exceeds 28 VDC. This limits the dissipation

Figure 4. Classification Current (Classes 2 & 3:

R34 = 69.8

Ω and R34 = 45.3 Ω).

Voltage (V)

0.80

0.60

0.20

0.40

0.00

0

2

4

6

8

10

12

Class 1

Class 2

Class 3

Detect Min

Detect Max

Detection

Voltage

Range

23.75 k

Ω

26.5 k

Ω

Voltage (V)

14

10

12

8

10

12

14

16

18

20

22

Class 1

Class 1 Max

Class 1 Min

Classification

Voltage Range

of the classification circuit to approximately 20 mW at

high input line (57 VDC) plus detection resistor (R31)

dissipation of 130 mW.

Voltage (V)

22

18

26

30

14

10

12

14

16

18

20

22

Class 2

Class 3

Class 3 Max

Class 3 Min

Class 2 Max

Class 2 Min

Classification

Voltage

Range